It is well known in polyester manufacture to increase the molecular weight of a polymer produced in a melt-phase polycondensation process by the treatment of solid particles of polymeric material at elevated temperatures in the presence of a flowing inert gas stream. The gas stream removes the products of the reaction from the particles and may contain glycols, oligomers and acetaldehyde amongst other compounds. The gas stream is typically circulated through the reactor containing the solid particles continuously and passes through a purification reactor in order to remove the organic species contained therein before it enters the SPP reactor. This process is described in EP-A-0660746, EP-A-1315769, and EP-A-0699471. The process is known as solid state polymerisation (SSP), solid state polycondensation (SSP) or solid phase polymerisation (SPP) and these terms should be taken to have the same meaning in this specification.
The oxidation catalyst used in the purification process is typically either platinum (Pt) or palladium (Pd) or a mixture of them, supported on a solid support material such as alumina. EP-A-0660746 describes a process for the purification of inert gas stream containing organic impurities from an SPP reactor comprising adding oxygen or gas containing oxygen to the gas stream and circulating the gas stream through a catalytic bed containing Pt or mixtures of Pt and Pd supported on an inert porous support at temperatures from 250 to 600° C. The process is characterized in that the quantity of oxygen used is stoichiometric with respect to the organic impurities or in such an excess that the gas at the outlet of the oxidation reactor contains up to 10 ppm of oxygen.